This invention concerns sewage treatment, and especially relates to an enhancement in a particular process used in the liquid side of a wastewater treatment plant.
Sewage treatment plants in some locations and applications are required to remove phosphorus from the influent waste stream, to meet a maximum effluent standard for phosphorus (phosphorus as used herein is intended to include compounds of phosphorus as typically present in wastewater). This is in addition to nitrogen removal requirements. One conventional liquid-side process for removing both nitrogen and phosphorus would include an anaerobic stage followed by an anoxic stage, and then an aerobic stage. Recycling of a portion of the mixed liquor would occur from the aerobic stage back to the anoxic stage, and also from the anoxic stage back to the anaerobic stage to maintain bacterial population there. In this way, phosphorus can be released in the anaerobic stage, into the wastewater; and in the anoxic stage, nitrogen gas is released as microbes obtain oxygen from nitrates. In the aerobic stage, nitrification occurs and biological phosphorus uptake is promoted by the aerobic conditions. The recycling returns nitrates to the to the anoxic stage for nitrogen release and maintains bacterial population in the anaerobic stage. Typically, sludge is also recycled from the solids side of the plant, i.e. from secondary clarification, back to the anoxic stage. This recycle loop is for the purpose of retaining biomass in the system.
A relatively new liquid-side process has been used recently, known as the SymBio treatment process licensed by BioBalance of Denmark. The SymBio process conserves on tank volume by providing a combined aerobic/anoxic stage in which both nitrification and denitrification occur. This process, however, does not address phosphorus removal.
In the SymBio process, which is a continuous flow process, both aerobic and anoxic conditions exist in a single stage, within a single tank. Dissolved oxygen content is very limited, generally kept in a range below about 1 ppm by regulating aeration between a minimum and higher rates as needed for the process. A metabolic coenzyme of the bacteria in the aerobic/anoxic stage is monitored using ultraviolet light. The detectable coenzyme condition occurs when the microbes start to go anaerobic, which typically happens in the middle of a floc, even when outer regions of the same floc are aerobic or at least anoxic (in the anoxic condition nitrates are available to provide oxygen to the microbes). Dissolved oxygen is increased, as by aeration, when this anaerobic condition occurs, preventing the entire stage from becoming anaerobic. The development of the anaerobic condition can also be detected using chemical measurements of ammonium, phosphate, nitrates and volatile fatty acid concentration, measurement of redox potential, pH, alkalinity short-term measurements of BOD, COD, measurement of CO2 concentration, measurement of fluorescence emission from at least one characteristic biogenic fluorophore selected from the group consisting of NADH, NAD(P)H, tryptophanxe2x80x94and tyrosine-containing proteins, tryptophanxe2x80x94and tyrosine-containing peptides, tryptophanxe2x80x94and tyrosine-containing derivatives of amino acids, purines, pyrimidines, nucleosides, nucleotides, nucleic acids, steroids and vitamins, and combination thereof. The SymBio process is described in U.S. Pat. No. 5,906,746, incorporated herein by reference.
It is an object of this invention to modify the SymBio process to effect significant phosphorus removal, thus meeting effluent requirements in most cases for both nitrogen and phosphorus.
The following U.S. patents are believed to have background relevance to this invention: U.S. Pat. Nos. 4,159,945, 4,051,039, 3,709,346, 3,964,998, 3,733,264, 3,864,246, 3,994,802, 4,056,465, 4,162,153, 4,257,897, 4,271,026, 4,488,967, 4,488,968, 4,522,722, 4,522,663, 4,556,491, 4,650,585, 4,867,883, 4,402,493, 4,141,822, 4,183,808, 4,956,094, 4,874,519, and 3,966,965.
Under the present invention, it was noticed in operating the SymBio liquid side treatment process that when the microbes toward the middle of flocs in the aerobic/anoxic stage start to turn to the anaerobic condition, if this condition is allowed to persist for a sufficient time, this will provide a sufficient anaerobic condition in the treated liquid and bacteria to cause release of significant quantities of phosphorus into the liquid. The subsequent addition of dissolved oxygen in this stage will promote the desired biological phosphorus uptake. However, without a subsequent aerobic stage beyond the anoxic/aerobic stage, it was discovered, the biological phosphorus uptake will not be maximized and much of the released phosphorus will remain in the wastewater and in the plant effluent.
In accordance with the invention, the aerobic/anoxic stage as described above is closely monitored, and when microbes toward the center of flocs start to turn to the anaerobic condition, this is allowed to persist and develop for a preselected period of time, such as approximately five minutes, before the dissolved oxygen content is increased. This preselected time duration is chosen to balance the desire for development and maintenance of an anaerobic condition within the floc, for as much phosphorus release as possible, against the need to avoid a more widespread anaerobic condition within the stage, which will cause release of malodorous gases.
In addition, a small aerobic stage is added downstream of the aerobic/anoxic stage, provided to maximize the biological phosphorus uptake by the bacteria. As noted above, the re-aeration that occurs in the aerobic/anoxic stage will cause at least some of the microbes to take up phosphorus, but this effect is only partial in the absence of a further aerobic stage.
In the aerobic/anoxic stage according to the invention, dissolved oxygen level is maintained preferably in the range of about 0.01 to 0.99 ppm. In the subsequent aerobic stage, dissolved oxygen is maintained preferably between about 0.5 and 5.0 ppm.
In a variation of the invention, the small aerobic stage includes a membrane separator which efficiently removes nearly pure water from the solids in the aerobic stage. This can be a substitute for a clarifer downstream, with the removed water leaving the plant. The aeration in the tank with the membranes provides agitation which prevents clogging of the membranes.
As a result of the innovations described above, phosphorus removal can be achieved in the liquid side of a wastewater treatment process along with nitrogen removal, to an extent as to be acceptable for most applications requiring phosphorus removal. As compared to typical previous plant design for removing both phosphorus and nitrogen, far less tank volume is required, effecting a savings in square footage as well as cost, a savings that can be increased if a membrane separator is used in lieu of a clarifer. The aerobic stage can be contained in a relative small tank, with the volume ratio between the aerobic/anoxic stage and the aerobic stage being 1:1 or greater, even up to about 95:5. As compared to the known SymBio process, the invention additionally achieves phosphorus removal at relatively low additional cost.
Due to the reduction in aeration requirements with the inventive system, a reduction in off-gas production can result, and thus less odor emanates from a plant.